Braking Automobiles

Eye Opener presented by Porter Johnson
01 July 1999 Math-Phys SMILE

Materials:

Toy car with front and rear axles that rotate
independently--preferably a big plastic one about 12" [30 cm] long,
so that everybody in the class can see it.

Modeling clay.

First, roll the car across the floor [floor tiles are better for this
purpose than floor carpet], and see that it goes on a virtually straight
path. Then, use the clay to lock the rear wheels to the auto frame, so
that the rear wheels will not rotate. When the car is then rolled across
the floor, it tends to "fish tail" out of control, and in fact if there is
enough room the car will rotate so that the rear of the car ends up in
front.

In other words, if the rear wheels are locked and the front wheels are not,
the car is unstable and likely to spin around and out of control. To
illustrate this point, push the car across the floor with the rear end
[locked wheels] in front, and observe that the car is stable when going
backwards.

As a variation, you can lock the front wheels with the clay, and observe
the car goes forward rather well, but tends to "spin out" when going
backwards.

The effect is caused by the fact that the rolling front wheels are
instantaneously at rest with respect to the ground at the point of contact,
whereas the rear wheels are sliding over the ground [kinetic friction is
present]. Kinetic friction is always less than static friction, and the
force/torque imbalance produces an instability. For essentially the same
reason, you get more traction [on ice, snow, mud, or whatever] if you avoid
spinning the wheels of the car.

This exercise illustrates several points in automotive engineering:

Front wheel drive cars are intrinsically more stable than rear wheel
drive cars, since the power provided ot the front wheels always works
against a "spin out", whereas rear wheel drive cars ten to be more unstable.
Of course, for most of this century of the automobile, front wheel drive
cars did not exist!

It is important that the front wheels lock before the rear wheels do,
because otherwise the car will probably spin out of control.

ABS [automatic braking sensor] systems prevent the wheels from locking,
by monitoring and intervening when such a thing is about to happen.